This invention relates to orthotics, to supports or stabilizers for joints and, in particular, to a knee brace which serves both preventive and remedial functions in protecting against medial-lateral, anterior-posterior and rotary instabilities.
The knee joint is perhaps the most susceptible to injury of the major articulated joints of the human body, despite the presence of five major ligaments and two menisci which serve to connect and stabilize the tibia and femur. These anatomical structures include the anterior and posterior cruciate ligaments, the medial and lateral collateral ligaments, the posterior capsule ligament and the medial and lateral menisci.
Anatomically, the knee is designed so that specific muscles or muscle groups, not ligments, absorb the brunt of external or internal forces. That is, a muscle or group of muscles substitutes for each ligament in the knee to absorb force and resstrict motion. As examples, the hamstrings substitute for the anterior cruciate ligament, the quadriceps for the posterior cruciate liagment, and the abductor and adductor groups for the medial and lateral collateral ligments.
The articulation of the knee joint, and the ligaments, muscles and bones associated with the joint are described, for example, in Gray's Anatomy and in The Johns Hopkins Atlas of Human Functional Anatomy, 2d ed., 1980. These publications are incorporated herein by reference.
When a muscle is unable to completely absorb an applied force, either because of inherent weakness or prior injury or simply because the force is too strong, the unabsorbed component of force is transmited to one or more ligaments. If the transmitted component is sufficiently great, the ligament is strained or torn. Ligamental susceptibility to injury is also dependent upon the degree of flexion or extension. The inherent cooperation and relationship among the ligaments is such that when the knee is bent or flexed, some ligaments are relatively tight and tend to control displacement, but others are relatively loose. Between 20.degree.-60.degree. of flexion, the knee is very susceptible to displacement and to injury. This is unfortunate, because the knee is frequently in this position, particularly during the more active sports activities.
It is factors such as these which make the knee relatively weak compared to the other major articulated joints. Some, such as the relatively simple ball and socket hip joint, are very secure. Other, such as the elbow and shoulder joints are complicated but nonetheless relatively secure. Despite its inherent weaknesses, the knee joint must both support the weight of the body and provide for movement, while holding the tibia and femur in position along their substantially planar unstable interface.
In considering external forces applied to the knee and the resulting ligament injuries, it is helpful to simplify the situation somewhat and consider the forces as having their major components applied primarily along a frontal plane through the knee, or along a sagittal plane through the knee, or as comprising a rotatory force. Frontal plane forces are medial-lateral forces which displace the femur and/or tibia in a side-to-side direction. Saggital plane forces are anteriro-posterior forces which displace the femur and tibia in approximately a front-to-back motion, and includes drawering forces applied during flexion or extension. Rotatory forces are those which tend to induce relative rotational displacement of or between the femur and tibia, primarily against the stabilizing force provided by the anterior cruciate ligament.
FIGS. 1 through 3 illustrate examples of the above forces. In these schematic drawings, the femur, tibia and knee are respectively designated 11, 12 and 13. Referring specifically to FIG. 1, two of the more common knee ligament injuries result from medial and lateral forces. The lateral collateral ligament and medial collateral ligament are primary stabilizing influences against medial and lateral force, respectively. As a consequence, strains or tears of lateral collateral ligament and the medial collateral ligament tend to result, respectively, from medial forces, that is, inward or medially-directed forces 14 applied aginst the outside of the leg, or lateral forces 15, which are outward directed forces applied against the inside of the leg.
FIG. 2 depicts a posterior force 21, i.e., a rearward-directed force. The knee 13 is stabilized against posterior forces primarily by the posterior cruciate ligament. The posterior cruciate ligament is backed by the posterior capsule ligament, which is quite effective in stabilizing the knee against displacement. As a result, isolated posterior cruciate tears are rare. Usually injuries to other ligaments are also involved. In fact, it is not infrequent that the bone attachment itself tears rather than, or in addition to the posterior capsule ligament.
Perhaps the most frequent injury in sports, and certainly one of the most damaging injuries to the joints involves strains or tears of the anterior cruciate ligament (also termed the "ACL"). Referring to the side view shown in FIG. 3A, the responsible force may involve an anterior tibial force alone, that is, a forward-directed force 31 applied to the back of the tibia. This anterior force an result from a direct blow, such as hyperextension or a fall. As shown in FIG. 3B, the force may involve a rotational tibial force 32 which rotates the tibia relative to the femur (as by catching a ski, or by catching a shoe or cleats in turf) or the combination of a rotatory force with an anterior tibial force 31. The injury is a direct result of excessive force which the substitutional muscles and the anterior cruciate ligament are unable to absorb and a resulting anterior tibial acceleration and displacement relative to the femur. See FIG. 4.
The reason for the frequent occurrence of injuries to the anterior cruciate ligament is two-fold, namely the frequency with which the knee and leg are subjected to large magnitude forces, and the susceptibility to injury such as tearing in that typically the knee can withstand only about 380 pounds of force and 12.5 millimeters displacement or movement between the tibia and the femur without injury to the anterior cruciate ligament. If the knee happens to be flexed at the time the forces are applied, the probability of serious ACL injury becomes even greater. Of course, if there is existing damage or if the anterior cruciate ligament has inherent instability, the knee is more susceptible to displacement and the ligament is more susceptible to injury. The same is true of the other ligaments.
Any effective knee orthosis should be designed to provide three protective functions for the anterior cruciate ligament. First, it should restrict the movement of the tibia, absorbing the anterior force and eliminating any ACL damage or tear before it occurs. Second, it should stabilize the already weak or injured ACL against further injury to itself and, third, it should also provide decreased risk of injury to secondary structures sucha s the medial collateral ligament or the medial meniscus where the ACL is weak or injured. Thus, in addition to preventing an injury initially, effective knee orthosis may permit continued or resumed participation in regular athletic activities where there is an existing injury.
Concentrated efforts by the orthotics profession to develop knee stabilizers are thought to have been initiated in the 1960's as a result of publicized knee injuries suffered by professional athletes. It is believed basically two types of knee braces have dominated this field. Referring to the FIG. 5 front view, one such brace 50 uses a three-point pressure which is provided by two pads 51 and 52 situated above and below the knee (on either the medial or the lateral side of the leg) and a third pad 53 on the opposite side of the leg adjacent the knee. Rigid braces 54-54 connect the pads. Various straps can be used to enhance suspension and/or stabilization characteristics.
Referring to the side view shown in FIG. 6, the second type 60 of conventional knee brace uses relatively rigid anterior femural and tibial shells 61 and 62 which are joined by hinged uprights 63-63 and are supported in the back or posterior side by elastic straps 64 and 65. These designs are more effective at protecting against medial-lateral forces than anterior-posterior forces. The reason is simple. The rigid shells/pads and connecting braces provide relatively inflexible pressure points which stabilize against lateral or medial forces.
In contrast, the relatively flexible front-to-rear stabilization systems provided by the braces 50 and 60 permit relative movement of the tibia and femur along the sagittal plane.
In addition, because rotary stability is a function of both medial-lateral and anterior-posterior stability, the implementation of conventional knee brace designs tends to be less effective than desired in any derotation function. Furthermore, stabilization in all aspects is closely related to the effective suspension of the orthotic device on the knee and leg in a manner such that the device does not alter or shift its position on the leg as by planing. Many prior art devices experience planing and shifting which detract from their ability to provide medial-lateral stability, anterior-posterior stability and/or rotatory stability.
Frequently, designers of prior art knee braces have attempted to avoid the problems associated with flexion as well as suspension and stability by simply encasing the knee or leg in a rigid, heavy brace, using considerable strapping, which restricts movement of the knee. Because of restrictions on movement and because of weight, such braces put the athlete at such a competitive disadvantage that these knee braces are not widely used for injury prevention. Rather, the primary use has been compensatory or remedial, that is, to compensate or stabilize knees with one or more weak or injured ligaments (such as the anterior cruciate ligament) and to prevent injury to other structures in knees in which ligaments are already torn or attenuated. Perhaps the only exceptions to the use of prior art knee braces for remedial purposes rather than prevention are the braces depicted in FIGS. 5 and 6, as well as the class of braces which consist simply of a pair of upright bands on the sides of the leg. However, this latter type of brace is used to provide some means of protection against medial-lateral forces, not anterior-posterior forces and, as mentioned above, the former two types are not wholly effective in protecting against injuries to the anterior cruciate ligament.
In short, a knee orthosis can serve any of three crucial purposes related to the anterior cruciate ligament: to prevent anterior cruciate ligament injury, to stabilize a knee having a weak or injured anterior cruciate ligament, and to prevent further injury to secondary structures when the anterior cruciate ligament has already been torn or attenuated. However, as discussed above, because of (1) the large forces which are applied to and generated by the leg and (2) the very small displacement which is possible without injury to the anterior cruciate ligament, it has proven very difficult to accomplish these goals of stabilization and, in particular, of prevention. Furthermore, the requirements of light weight and comfort and freedom of movement are, seemingly, in direct conflict with the goals of suspending the brace on the leg without relative movement relative to the leg and restricting the displacement of the anterior cruciate ligament. Probably because of these conflicting requirements, with the exception of the braces disclosed in my above two copending applications, known braces are believed to be designed to provide light weight and freedom of movement at the expense of adequately stabilizing the knee, or to rigidly encase the knee at the expense of light weight and freedom of movement.